Determining underwater auditory thresholds in fish based on particle motion.

Abstract

Acoustic particle motion is believed to play a significant role in the auditory scene of fishes even though most all hearing sensitivity data are measured with respect to sound pressure level. Auditory motion thresholds have been measured in air for goldfish (Carassius auratus), oscars (Astronotus ocellatus), and toadfish (Opsanus tau) by moving the fish’s body using a carefully calibrated shaker table [R. R. Fay, Science 225, 951–954 (1984); Z. Lu, A. N. Popper and R. R. Fay, J. Comp. Physiol. A 179, 227–233 (1996); R. R. Fay and P. L. Edds‐Walton, Hearing Res. 111, 1–21 (1997)]. These responses depend on both direction and amplitude of the motion. Thus, underwater particle motion thresholds must include both direction and amplitude, but most experimental setups are not designed for this purpose. Passive and active methods to control the direction and amplitude of acoustic particle motion underwater are presented. It is not enough just to measure the local particle motion in a small tank and report its amplitude as a threshold, especially when incidental sound pressure could be contributing to the auditory response and the orientation of the fish with respect to the direction of the particle motion vector is ill defined.